Bone fixation plate

ABSTRACT

An apparatus for reducing the profile of a bone fixation plate while, preventing backing out of screws is disclosed. The apparatus includes at least one section of relief and sections of engagement. The plate has at least two openings though which two screws can pass through bony tissue. As the screw is tightened, it will begin to lag the plate to the bone. When the screw head interferes with the plate at the interference point, there is a slight resistance that insertion forces can overcome. When the screw is advanced further, it snaps into the sliding fit area and is allowed to move freely. The forces that cause the screw to back out from the plate are preferably not strong enough to pass the screw head back past the interference section. It may be desirable to include a set screw to help prevent backout.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application claiming priority toU.S. patent application Ser. No. 10/826,285 filed on Apr. 19, 2004 nowU.S. Pat. No. 7,963,981, the entire contents of which are incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to a bone fixation plate used to stabilizevertebrae and other bony anatomy. More specifically, the presentinvention relates to a cervical plate having a minimized profile thateasily and reliably prevents backout of fastening devices.

BACKGROUND OF THE INVENTION

Bones and bony structures are susceptible to a variety of weaknessesthat can affect their ability to provide support and structure.Weaknesses in bony structures may have many causes, includingdegenerative diseases, tumors, fractures, and dislocations. Advances inmedicine and engineering have provided doctors with a plurality ofdevices and techniques for alleviating or curing these weaknesses.

The cervical spine has presented the most challenges for doctors,partially due to the small size of the vertebrae and the spacing betweenadjacent vertebrae. Typically, weaknesses in the cervical spine arecorrected by using devices that fuse one or more vertebrae together.Common devices involve plate systems that align and maintain adjacentcervical vertebrae in a desired position, with a desired spacing.

These devices commonly referred to as bone fixation plating systems,typically include one or more plates and screws for aligning and holdingvertebrae in a fixed position with respect to one another. Initialdevices used stainless steel plates and screws. In order to remain fixedin place, the screws were required to pass completely through thevertebrae and into the spinal canal. These devices caused manycomplications and involved significant risks. To allow a screw to pass,drilling and then tapping of the vertebrae was required. In the process,instruments came within close proximity of the spinal cord, whichrequired extreme care on the part of the surgeon.

In addition to the risks of surgically applying bone fixation plates,other complications arose. Commonly, these problems involve looseningand failure of the hardware. Two common failures are the breakage of theplates, and the backing out of the screws into soft tissues of thepatient's body. The backing out of the screws is typically a result ofthe screws failure to achieve a sufficient purchase in the bone,although the stripping of the screws has also been known to cause thisproblem. Regardless of the cause of the hardware failures, a surgeonmust repair or replace the broken parts, which requires undesirableinvasive procedures.

Advances in material science allowed engineers to manufacture bonefixation plates out of materials that would resist breakdown within abody. However, the backing out of screws remained a problem. Manysolutions were devised in an attempt to prevent this from occurring. Oneprevalent solution involved minimizing the length of the screw in orderto prevent screw to plate junction breakage of the screw. However, theshortened screw is typically unable to achieve a sufficient purchase inthe bone. Shortened screws often provide very little holding power andinadequate tactile feedback to the surgeon. Tactile feedback to thesurgeon is important to signal completion of tightening prior tostripping of the screw within the bone.

An alternate solution involves increasing the length of the screws inorder to achieve sufficient purchase to hold the plate in place. Whilethe use of longer screws can provide bicortical fixation, this methodalso has its drawbacks. Primarily, long screws increase the chances ofinterference with each other when they are screwed into bony tissue atan angle. In addition, many bone fixation plating systems place bonegrafts between vertebrae. The bone grafts are eventually supposed tospur the growth of bone between the vertebrae, so that the vertebraebecome fused together naturally.

In order for this to occur, the bone fixation plating needs to maintaina desired spacing between the vertebrae, which is filled by the bonegrafts. However, it is common for the bone grafts to experiencecompression, which separates at least one of the adjacent vertebrae fromthe bone graft. Cervical plates that employ long screws do not allow forsufficient movement of the vertebrae to accommodate the compression ofthe bone graft, because the purchase of the screws is too great. Thus,the vertebrae cannot move and are unable to adjusting to the compressionof the bone graft.

Another method of preventing the backing out of screws involves placinga second plate over the screws. This second plate functions to interlockthe screws, preventing them from backing out. However, this method ofsecuring screws often becomes bulky, resulting in a large andundesirable profile. In addition, these configurations require carryingout multiple steps or using a multi-piece assembly in order to block anopening through which a loose fastener head may pass. For instance, theuse of a c-ring that can expand as the fastener head is insertedrequires additional components and assembly time to form a plate.Moreover, multi-component designs may lose their ability to retain afastener over time due to material failure, relaxation, or the like.Additionally, multi-component configurations may not provide sufficientability to lag the plate to the vertebral body.

One additional drawback of many designs is that they add to the overallheight of the plate. It is desirable to maintain a low profile designfor many reasons, such as to minimize irritation to surrounding tissue.For example a plate design having a high overall height or a receptacledesign that does not prevent screw backout may cause a patient to sufferfrom dysphasia. Ultimately, the screw or plate may irritate or wearthrough neighboring tissue. In addition, a high height plate orunretained loose screw in the lumbar spine may be abrasive to the aortaor vena cava. Severe abrasion by the plate or screw in this instance maypuncture the aorta or vena cava and cause internal bleeding.

In addition, many of these plates were not designed to allow for thelocking of all of the screws, which left some of the screws susceptibleto backout caused by tiny vibrations, or micromotion. Some methodsattempted to reduce the profile of the total system by using smallparts. However, this led to the small parts falling off and gettinglost. In addition, the smaller parts are fragile and require specialinstruments in order to insert or manipulate them. In addition, becauseof their small size, incorrect placement relative to the axis of theplate often causes sharp and jagged shavings to be formed as a lockingscrew contacts an improperly seated bone screw.

Prior attempts at increasing the screw purchase have resulted in riskyprocedures, or an insufficient ability to adapt to movement. Attemptsand decreasing the profile of bone fixation plates have resulted in lostparts, or insufficient purchase. A continuing need exists for anapparatus that is able to quickly and reliably lock a plurality ofscrews into place while maintaining a low profile.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus for connecting a plate toa bone. This may be desirable in order to immobilize, for example, twocervical vertebrae. In one embodiment, the present invention comprisesat least one screw and a plate having at least one opening. As the screwpasses through the opening and is tightened, it begins to lag the plateto the bone. When the screw head interferes with the plate at aninterference point, there is a slight resistance force that insertionforces can easily overcome. When the screw is advanced further, it snapsinto the sliding fit area and is allowed to move freely. Forces whichcan cause the screw to back out of from the plate are preferably notstrong enough to pass the screw head past the interference section. Insome embodiments, it may be desirable to use a set screw to aid inpreventing backout. Alternately, a clamp applied to the head of thescrew to prevent rotation may be desired.

In one embodiment, the present invention comprises an apparatus forfixing a plate to bony material, comprising at least one opening havinga spherical curvature. Also included is at least one fastener having ahead that interferes with the spherical curvature at an interferencepoint. In this embodiment, the head is capable of engaging with andpassing the interference point to communicate with the sphericalcurvature.

In some embodiments, the spherical curvature includes at least oneengagement area and at least one relief area. The tangents to thespherical curvature preferably intersect to form an angle. Preferably,the angle of intersection of the tangents is between about 1 and about 5degrees. More preferably, the angle of intersection of the tangents isbetween about 1 and about 3 degrees.

It is desirable to limit the relief areas in some embodiments to preventa screw from passing through the interference point. Accordingly, it ispreferred that the relief area comprises less than about 40% of thecircumference of the spherical curvature. More preferably, the reliefarea comprises less than about 30% of the circumference of the sphericalcurvature. In some embodiments, it may be desirable to provide anadditional opening that is configured and dimensioned to increase themagnitude of interference at the interference point.

In another embodiment, the present invention comprises an apparatus forstabilizing at least two bony structures, comprising a plate where morethan one aperture is configured and adapted to include an interferencearea. The interference area is integrally formed in the plate to preventa fastener from backing out of the interference area.

In this embodiment, a fastener, such as a screw, is capable of engagingwith and passing through the interference area. The interference area ispart of spherical curvature, which has at least one engagement area andat least one relief area.

Preferably, the tangents to the spherical curvature intersect. It isdesirable to have the angle of intersection of the tangents betweenabout 1 and 5 degrees. In some embodiments, it is also preferable toinclude another opening that is selectively positioned to increase themagnitude of interference at the interference point. The opening may beconfigured and adapted such that it is able to pass a wedge shapedscrew.

In another embodiment, the present invention comprises an apparatus forfixing a plate to bony material consisting essentially of at least oneopening having a spherical curvature. At least one fastener having ahead capable of engaging with and passing through an interference pointof the spherical curvature is also included. In this embodiment, thefastener is prevented from backing out of the opening by theinterference point. In this embodiment, the tangents to the sphericalcurvature intersect. As described above, another opening may beselectively positioned to increase the magnitude of interference at theinterference point.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one embodiment of the bone fixation plateaccording to the present invention;

FIG. 2 is a diagram showing a side view of exemplary openings accordingto the present invention;

FIG. 3A is a diagram showing one embodiment of the spring loaded plateaccording to the present invention;

FIG. 3B is a diagram showing an exemplary ramped surface included in thespring loaded plate of FIG. 3A;

FIGS. 4A and 4B are diagrams showing an exemplary embodiment of a setscrew according to the present invention;

FIGS. 5A and 5B are diagrams showing an exemplary embodiment of a bonescrew according to the present invention;

FIG. 6 is a diagram showing another embodiment of the bone fixationplate according to the present invention;

FIG. 7 is a diagram showing one embodiment of the spherical curvatureaccording to the present invention;

FIG. 8 is a diagram showing the forces exerted by the screws accordingto the embodiment shown in FIG. 6;

FIG. 9 is a diagram showing another embodiment of the bone fixationplate according to the present invention;

FIGS. 10A and 10B are illustrations of additional embodiments of bonefixation plates of the present invention;

FIG. 11 is a diagram showing a one embodiment of the bone fixation plateaccording to the present invention;

FIG. 12 is a diagram showing a drill guide in communication with a bonefixation plate of the present invention;

FIG. 13 is a magnified view of a drill guide in communication with abone fixation plate of the present invention;

FIG. 14 is a side view of a drill guide in communication with a bonefixation plate of the present invention;

FIGS. 15 A-C are diagrams showing an exemplary embodiment of a rigidbone screw according to the present invention; and

FIG. 16 is an illustration of one embodiment of a drill guide capable ofrotating about an axis of a receptacle or depression formed in theplate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a bone fixation plate that minimizesthe problems associated with prior bone fixation plates whilemaintaining a small profile. In one embodiment, as a screw is tightened,it will begin to lag the plate to the bone. When the screw headinterferes with the plate at an interference point, a slight resistanceis generated. The insertion forces can easily overcome this resistance.When the screw is advanced further, it snaps into a sliding fit area andis allowed to move freely. The forces which can cause the screw to backout from the plate are preferably not strong enough to pass the screwhead back past the interference section. It may be desirable to includea set screw to prevent backout of the screws due to micromotion. Inother embodiments, the head of the screw may be clamped to preventrotation, when such a restriction on the movement of the screw isdesirable.

The present invention provides a locking mechanism that allows one ormore bone screws used for attaching a plate to vertebrae to be easilyand reliably locked in place at the same time by a single operation.When fully installed, the locking mechanism has a low profile andmaintains its ability to prevent breakout of screws due to micromotion.The present invention may be used on the anterior or posterior of thevertebrae. Although the present invention is described with respect totwo bone fixation vertebrae, it will be understood that the followingembodiments are capable of being used with any number of vertebra, inany spinal location.

Turning now to the drawings, FIG. 1 shows one embodiment of a bonefixation plate 101 according to the present invention. The plate may besecured to two vertebrae in order to maintain the vertebrae integrallywith respect to one another in a desired orientation and at a desiredspacing from one another. Plate 101 includes two fastening devices, suchas screws 103-105 or the like, which are operatively communicable withspring loaded plates 107-109. The plate also includes four openings111-117, through which screws (not shown) may be used to fasten theplate 101 to the vertebrae.

The plate 101 and the screws may be comprised of any material, such as ametal, alloy, or any combination of the two. Preferably, the materialused to construct the plate and the screws allows the plate 101 tomaintain its structural integrity while allowing for a desired amount ofresiliency. Furthermore, the material used is preferably bio-compatibleand capable of withstanding the conditions of a body over a desiredperiod of time. In some embodiments, this is achieved by manufacturingthe plate 101 and screws using metals such as titanium or stainlesssteel. Titanium has sufficient ductility to permit a desired amount ofcurving of the plate 101 to conform to the shape of the vertebrae, yethas the strength to maintain its structural integrity.

In the FIG. 1 embodiment, the bone fixation plate 101 comprises a centerportion 119 and two distal portions 121-123. Each distal portion 121-123may be attached to a different vertebra using fasteners, such as screws,that pass through openings 111-117. Because distal portions 121-123 aresimilar, only the operation of distal portion 121 is described indetail.

FIG. 2 is a diagram showing a side view of openings 111 and 113. In oneembodiment, each opening has a substantially circular shape, as shown inFIG. 1. In this embodiment, the inner portion of openings 111-113 havesubstantially spherical curvatures. Accordingly, the radius of the innerportion of openings 111-113 decrease in diameter from the top 201 of theopenings, to the bottom 203 of the openings. Preferably, the sphericalcurvature of the openings 111-113 may accommodate a screw having aspherical head. However, the present invention is not limited tospherical curvatures or spherical heads. In other embodiments, anycomplementary head and receptacle may be used. Preferably, thecomplementary head and receptacle are capable of preventing the breakoutof the screw.

As shown in FIG. 2, the openings 111-113 are not continuous. It isdesirable that the openings 111-113 comprise only a portion of thecircumference of the spherical curvature. In one embodiment, theremaining portion 205 of the circumference of the spherical curvature ofthe openings 111-113 is provided by spring loaded plate 107, shown inFIG. 1. The portion of the circumference of the spherical curvature thatis completed by spring loaded plate 107 may be varied as desired, forexample, according to the amount of resistance that is desired by thespring loaded plate 107. In one embodiment, the openings 111-113comprise at least 60 percent or more of the total circumference of thespherical curvature. In another embodiment, the openings 111-113comprise at least 70 percent or more of the total circumference of thespherical curvature. In yet another embodiment, the openings 111-113comprise at least 80 percent or more of the total circumference of thespherical curvature.

FIG. 3A is a diagram showing one embodiment of the spring loaded plate107. In this embodiment, the spring loaded plate 107 includes arm 301.When a force causes arm 301 to be deflected towards the body 303 of thespring loaded plate 107, potential energy is stored in the arm 301. Thispotential energy causes the arm 301 to generate spring-like forces thathave a tendency to force it away from the body 303, and back to itsnatural resting position shown in FIG. 3A. When the deflection force isremoved, the potential energy is converted to kinetic energy, and forcesthe body 303 away from the arm 301. In other embodiments, the springloaded plate 107 does not have to have a free cantilever load such asthe arm 301 shown in FIG. 3A. For example, it may be desirable to use aloop, or the like, to resist movement of the spring loaded plate 107.

The inner portion of plate 107 preferably comprises a ramped surface305. In one embodiment, the ramped surface 305 is selectively engageablewith screw 103, shown in FIG. 1. When the screw 103, is engaged by theramped surface shown in FIG. 3B, outward forces are generated on thescrew, preventing it from backing out. As the angle of the rampedsurface increases, the forces that are exerted on the screw 103increase. Thus, the angle of the ramped surface may be chosen based onthe amount of force that is desired to keep the screw 103 from backingout.

In one embodiment, the angle of the ramp is between about 5 and 50degrees. In another embodiment, the angle of the ramp is between about10 and about 30 degrees. In yet another embodiment, the angle of theramp is between about 15 and 25 degrees.

The spring loaded plate 107 comprises two spherical curvatures 307 and309. Spherical curvatures 307 and 309 complete the spherical curvaturesof openings 111 and 113. Each curvature 307-309 comprises a sphericalcurvature having a radius that decreases from top to bottom, asdiscussed with respect to the curvatures of openings 111 and 113. Thespherical curvatures 307-309 may comprise any desired percentage of thecircumference of the total spherical curvature. In one embodiment, eachcurvature 307-309 may comprise 20 percent or less of the totalcircumference of the spherical curvature. In another embodiment, eachcurvature 307-309 may comprise 30 percent or less of the totalcircumference of the spherical curvature. In yet another embodiment,each curvature 307-309 may comprise 40 percent or less of the totalcircumference of the spherical curvature.

Spring loaded plate 107 also includes two edges 311 and 313, shown inFIG. 3A. Each edge is preferably configured and dimensioned to beengageable with a depression 125 in plate 101. In one embodiment, thespring loaded plate 107 is positioned within the depression 125.Depression 125 is configured and dimensioned such that there issufficient space for plate 107 to move between its compressed andrelaxed states, described with respect to FIGS. 3A and 3B. In oneembodiment, plate 107 is prevented from horizontally exiting depression125 by the protrusion formed by openings 111-113.

In one embodiment, shown in FIGS. 4A and 4B, the screw 103 may have anangled head 401. It may be desirable for screw 103 to have threads alongits elongate shaft 403. In order to aid in tightening screw 103, itpreferably includes a projection with a curved surface to aid ingripping the screw. The length of the elongate shaft may be varied asdesired. In one embodiment, the length of the elongate shaft is about 5mm or less. In another embodiment, the length of the elongate shaft isabout 3 mm or less. In yet another embodiment, the length of theelongate shaft is about 1 mm or less.

FIGS. 5A and 5B are diagrams showing one embodiment of the screw that isused to connect plate 101 to vertebrae. Screw 501 preferably has aspherical head 503 that is selectively engageable with the sphericalcurvature. An elongate shaft 505 is connected to the spherical head 503to allow it to penetrate bony tissue of the vertebrae. Preferably, theelongate shaft 505 includes threads that aid in fixing the plate 101 toa vertebra. As shown in FIG. 5B, it is desirable to have a hexagonalprojection 507 to aid in gripping the screw.

The length of the elongate shaft 505 may be varied as desired. In oneembodiment, the length of the elongate shaft is about 20 mm or less. Inanother embodiment, the length of the elongate shaft is about 10 mm orless. In yet another embodiment, the length of the elongate shaft isabout 5 mm or less.

In one embodiment, screw 103 is inserted into a receptacle in depression125. It is desirable to have a threaded receptacle such that the screwis capable of being fixed to the plate 101. The screw 103 also passesover plate 107, and prevents it from vertically exiting depression 125.The placement of the screw receptacle is preferably chosen such that itis engageable with the ramped surface 305 of plate 107 when the plate isin its relaxed state, with its arm 301 extended.

Preferably, two screws 501 are inserted into openings 111 and 113. Asthe screws 501 are tightened, they will begin to lag the plate 101 tothe bone. When the screw head 503 interferes with plate 107, it forcesit to move towards the center of the plate 101. As the screws 501 areadvanced further, the plate 107 forces its way back into its relaxedstate. This causes the spherical curvatures 307-309 to form a completespherical curvature around the screw head 503. When plate 107 is in itsrelaxed state, it prevents screw 501 from backing out. It may bedesirable to tighten screw 103, such that plate 107 remains fixed in itsrelaxed state. In this manner, the screw 501 is prevented from backingout.

Screws 501 may be screwed into bony tissue at any desired angle. Inother words, screw 501 does not have to be inserted perpendicular to theplate 101. The spherical properties of the head of the screw 503 and thespherical curvature of the openings 111-117 are preferably sufficient toprevent the screw from backing out. Thus, the largest diameter of thehead of the screw is larger than the diameter of the narrowest portionof the opening in the top our outer side of the plate through which thescrew head is placed. The interference difference between the fastenerhead diameter and the outer narrow opening may be describe in differentways depending on the size of the plate, openings, and fastener headsbeing used. For example the interference difference between the fastenerhead and the narrowest opening may be about 0.01 mm or greater, about0.03 mm or greater, or about 0.10 or greater, or even about 0.20 mm orgreater. Preferably, however, in each instance the interference is lessthan about 2 mm.

Alternatively, the interference between the fastener head and the narrowouter opening may be described relative to the outer diameter of thefastener head itself. For example, the interference may be about 0.5% orgreater of the diameter of the fastener head, about 5% or greater of thediameter of the fastener head, or even about 10% or greater of the outerdiameter of the fastener head. Preferably, however, in each instance theinterference is less than about 40% of the outer diameter of thefastener head.

While openings 111-117 prevent the screws 501 from backing out, they doallow it to rotate freely within the spherical curvature. One advantageof allowing the screw 501 to rotate freely is that the bone fixationplate according to the present invention is able to accommodate formovements in the vertebrae or accommodate for compression of the bonegrafts that are placed between vertebrae. Another advantage of allowingthe screws to be inserted at any angle is that it allows relativelyclose spacing of the screws, without the risk of interference with oneanother.

FIG. 6 shows another embodiment of the present invention. As shown inFIG. 6, an exemplary bone fixation plate according to the presentinvention comprises four openings 601-607. In one embodiment, openings601-603 are connected to one vertebra, and openings 605-607 areconnected to a second vertebra. Also included are two additionalopenings 609-611, which are located at a desired point between points601-603 and 605-607, respectively. One advantage of the FIG. 6embodiment is that a screw does not have to be inserted into opening 609until after screws are inserted into openings 601-603. Thus, opening 609serves as a window for a surgeon to view the vertebra, or space betweenadjacent vertebrae. This is often desirable to the surgeon.

Because all of the corresponding openings are similar, only openings601-603 and 609 are described in detail. In one embodiment, openings 601and 603 are spherical curvatures having the same properties discussedwith respect to FIGS. 1-5. Thus, a complete description of the openings601-603 is not repeated. The openings are substantially similar in size,shape, and diameter to the openings 111-117 described with respect toFIG. 1. There are some differences between the openings shown in FIG. 6embodiment and the openings shown in the FIG. 1 embodiment which, arediscussed below.

In one embodiment, the spherical curvature of openings 601-603 issubstantially circular. The opening 601-603 comprises the majority ofthe circumference of the spherical curvature for the screw 613. This isin contrast to the spherical curvatures described with reference toFIGS. 1-5, which were formed by both the openings and the spring loadedplate 107. Therefore, the spherical curvature of each opening 601-603houses substantially entire head of screw 613. In one embodiment, screw613 is substantially similar to screw 501, discussed with reference toFIGS. 5A and 5B.

In one embodiment, the spherical curvature of the opening 601-603comprises 90% or more of the total circumference of the curvature. Inanother embodiment, the spherical curvature of the opening 601-603comprises 95% or more of the total circumference of the curvature. Inyet another embodiment, the spherical curvature of the opening 601-603comprises 99% or more of the total circumference of the curvature.

In one embodiment, opening 609 is selectively positioned betweenopenings 601-603. Opening 609 preferably allows a screw 615 to pass,which increases the interference between the spherical curvature of theopening 601-603 and the head of the screw 613. As shown in FIG. 6, theplacement of the openings 609-611 may be varied. In one embodiment, theopening may be positioned such that it positioned directly in betweenthe openings or slightly higher than the openings. However, in anotherembodiment the opening 609-611 may be placed at a desired distance abovethe openings.

In order to use screw 615 to cause tighten the openings 601-603 aroundthe head of screw 613, openings 601-603 comprise a fixed portion and aflexible portion 617. In one embodiment, flexible portion 617 is formedby a discontinuity that is formed in openings 601-603 and opening 609.When the flexible portion 617 of the opening is pushed against the screw615, increased interference results. One advantage of the increasedinterference is that backout of the screw 615 is prevented.

The discontinuity 619 should be large enough that it allows flexure ofportion 617 of the spherical curvature, while allowing the sphericalcurvature to maintain its structural integrity and provide a sufficientcontact area for the head of the screw 613. In one embodiment, thediscontinuity 619 shown in FIG. 6 comprises a small portion of the totalcircumference of the opening 601-603. The discontinuity 619 may bevertical, or it may be configured and dimensioned at a desired angle.

In one embodiment, the discontinuity 619 comprises about 5% or less ofthe total circumference of the curvature. In another embodiment, thediscontinuity comprises about 3% of less of the total circumference ofthe curvature. In yet another embodiment, the discontinuity comprisesabout 1% or less of the total circumference of the curvature.

In the FIG. 6 embodiment, tangents to the curvature of opposing pointsalong the spherical curvature intersect. This is in contrast to typicalspherical curvatures that have been used for bone fixation plates, wherethe tangents to the curvature of opposing points do not intersect. Oneadvantage of having the tangents to the curvature intersect is that thespherical curvature generates an interference area. As the head of thescrew being screwed into place, a sufficient amount of force may beapplied to force the head of the screw to contract slightly. As thescrew is continues into the bone, the head of the screw is able to passthrough the interference area. Once the head of the screw passes throughthe interference area it fits into the spherical curvature. It isdesirable that forces that force the screw to backout are not strongenough to force the screw back through the interference area. Thisresistance of the interference area may be modified by changing itscurvature.

In one embodiment, tangents to spherical curvature intersect to form anangle, as shown in FIG. 7. This angle is preferably between about 1 andabout 10 degrees. In another embodiment, the angle between the tangentsis between about 1 and about 5 degrees. In yet another embodiment, theangle between the tangents is between about 1 and about 3 degrees.

In one embodiment, opening 609 is substantially similar to openings601-603. That is, it has a substantially spherical curvature. In otherembodiments, however, opening 609 may not have a substantially sphericalcurvature. The curvature may be shaped to receive a screw 615 having aflat head. However, other types of screws may be used. In embodimentswhere screw 615 has threads, the receptacle may be configured to receivethe threads in order to prevent screw 615 from backing out. To allow aninstrument to grip the screw, a hexagonal depression may be configuredon the head of the screw. However, in other embodiments it may bedesirable to have a curved protrusion to aid in gripping the screw 615.

The diameter of opening 609 is preferably smaller than the diameter ofopenings 601-603. The diameter of opening 609 may be smaller than thediameter of openings 601-603 because the screw 615 that passes throughthe opening does not have to pass through bony tissue. In oneembodiment, opening 609 and screw 615 function to further restrictopenings 601-603 after the screw 613 has been inserted.

Screw 615 may be screwed into bony tissue at any desired angle. In otherwords, screw 615 does not have to be inserted perpendicular to theplate. The spherical properties of the head of the screw 615 and thespherical curvature of the openings 601-603 are preferably sufficient toprevent the screw from backing out. While openings 601-603 prevent screw613 from backing out, they do allow it to rotate freely. One advantageof allowing the screw 613 to rotate freely is that the bone fixationplate according to the present invention is able to accommodatemovements in the vertebrae or accommodate for compression of bone graftsthat may be placed between vertebrae. Another advantage of allowing thescrews to be inserted at any angle is that it allows relatively closespacing of the screws, without the risk of interference with oneanother.

As described above, openings 609-611 may be placed in any desiredposition. In some embodiments, it may be desirable to position theopening 611 directly in line with openings 605-607. However, in otherembodiments it may be desirable to place the opening 609 at a higherposition than the openings 601-603. FIG. 8 is a diagram showingexemplary forces that may be exerted on the openings 601-607 when screwsare inserted into openings 609 and 611.

As shown in FIG. 8, when the opening 609 is positioned at a higherposition than openings 601-603, the screw exerts forces on parts 609-611of the openings. Because the magnitude of interference between theopenings 601-603 and the screw 613 is not as great, this embodiment maybe preferable in applications where a significant amount of movement orshifting of the vertebrae is expected. The lower magnitude ofinterference allows the screws to shift to accommodate these movements.However, when the opening is positioned directly in line with openings605-607, the screw exerts forces on a larger portion of the openings605-607. Because of the increased magnitude of interference between theopenings 605-607 and the screw 613, this embodiment may be desirablewhen it is preferable to have the plate held in place with more force.

FIG. 9 is a diagram showing another embodiment of the present invention.In this embodiment, the present invention comprises at least twoopenings through which screws may pass. The screws used in thisembodiment are similar to the screws described with respect to FIGS.1-8, thus a discussion of them is not repeated. In some embodiments, athird opening may be selectively positioned between the at least twoopenings in order to prevent a screw from backing out of the openings.Though only one set of openings is shown in FIG. 9, a corresponding setof openings are also attached to an adjacent vertebra. Adjacent sets ofopenings are preferably connected by two elongate shafts.

As shown in FIG. 9, openings 901-903 comprise spherical curvatures, asdescribed with reference to FIGS. 1-8. As described with reference toFIG. 7, tangents to the spherical curvature intersect to form an angle.The angles are similar to those discussed with reference to FIG. 7, andtherefore are not repeated. In addition to the spherical curvaturedescribed with reference to FIG. 7, the FIG. 9 embodiment also includesalternating sections of engagement 905 and sections of relief 907. Thesesections of engagement 905 and relief 907 are preferably located alongthe top portion of the spherical curvatures, from which the screw isinserted. One advantage of having one or more sections of engagement andrelief is that the openings 901-903 are able to accommodate micromotionof the screw, or in some cases, of the entire plate.

In one embodiment, the openings 901-903 comprise a single relief section907. This provides the advantage of allowing a screw to adjust due tomicromotion, while preventing the screw from backing out. In thisembodiment, the remainder of the openings 901-903 is a section ofengagement. The section of engagement preferably resists the motion ofthe screw.

In another embodiment, more than one section of relief 907 may beincluded. More than one relief section 907 may be desirable inembodiments where micromotion may be prevalent. The sections of relief907 allow the angle of the screws to vary while preventing it frombacking out. However, it is undesirable to include too many sections ofrelief 907. It is desirable to have more sections of engagement 905 thansections of relief 907 because too many sections of relief will resultin the magnitude of the interference point being reduced at differentangles. Thus, in a preferred embodiment, the openings 901 and 903include more sections of engagement 905 than sections of relief 907.

In one embodiment, the number of relief sections 907 included in theopenings is two or greater. In another embodiment, the number of reliefsections that are included in the openings is four or greater. In yetanother embodiment, the number of relief sections that are included inthe openings is six or greater.

In some embodiments, the portions of the opening that are reliefsections 907 and the portion of the openings that are engagementsections 905 may be expressed as a percentage of the total circumferenceof the openings 901-903. Preferably, the sections of relief compriseabout 50% or less of the circumference of the openings. More preferably,the sections of relief comprise about 40% or less, and most preferablythe sections of relief comprise about 30% or less of the circumferenceof the openings.

In one embodiment, a third opening 913 may be placed between openings901-903. A set screw may be placed in opening 913 in increase theinterference of the openings 901-903 against the head of the screw. Inorder to allow the screw to increase the interference between theopenings 901-903 and the screw, a wedge shaped depression may beconfigured and dimensioned in the plate. The FIG. 9 embodiment providesthe advantage of minimizing the profile of the bone fixation plate whileincreasing its ability to accommodate for micromotion.

In this embodiment, as a screw is tightened, it will begin to lag theplate to the bone. When the screw head interferes with the sphericalcurvature at an interference point, a small amount of resistance isgenerated. The interference, and resulting resistance, are caused by theangle of intersection of the tangents to the spherical curvature. Asdescribed above, the spherical curvature has tangents that intersect.The interference forces are easily overcome by the screw head. When thescrew advances further, it snaps into the spherical curvature and isallowed to move freely. The forces which cause the screw to back outfrom the plate are preferably not strong enough to pass the screw headback past the interference section. To further assure that the screwhead does not pass back past the interference section, the set screwdescribed above may be employed.

Referring now to FIGS. 11-14, the plate of the present invention may beconfigured to aid in the insertion of bone screws. For example, FIG. 11illustrates that the plate may have one or more openings 1101 that arecapable of securely receiving a drill guide. For example, the openingsmay be configured with threads that engage with a threaded tip of thedrill guide. In addition, the plate may also have one or more recesses,pivot points, depth stops, or areas of removed material in the topsurface of the plate that help align the drill guide opening over theholes of the plate. The drill guide may have a rotating barrel thatrotates along an axis that extends through the recess of the plate. Inone embodiment, a portion of the drill guide can be aligned with andcontact the recess while providing a base on which the barrel canrotate. Alternatively, as shown in FIG. 16, a portion of the barrelitself may reside in the recess of the plate upon which the drill guidemay be rotatably disposed.

As shown in FIG. 13, the barrel may have a drill bore extending throughits length. When the drill guide is properly aligned with the recess andopening, the barrel may be rotated to a first position such that thedrill bore is aligned over a hole in the plate where a bone fastenerwill be placed. Preferably, the bore is configured so that its axispasses through the spherical opening in the plate. The portion of bonebeneath the plate may then either be prepared for receiving a fastenerby drilling a pilot hole in the bone, or alternatively a fastener may beplaced directly into bone. To further ensure that the fastener isinserted at a proper angle, it may be inserted through the bore.

Once a first fastener has been inserted into a first hole of the plate,the barrel may be rotated such that it is aligned over a second hole inthe plate, thereby allowing a second fastener to be inserted withouthaving to reposition the entire drill guide. As shown in FIGS. 11-14, aplurality of guide holes and recesses may be provided in the plate. Inone embodiment, one recess and guide hole may be used to insertfasteners into two bone screw holes.

As discussed above, once a fastener head has passed the interferencearea it may freely swivel or rotate to accommodate different angles orto allow for reabsorption of graft material over time. As graft materialor bone is reabsorbed by the body, loading previously borne by the bonemay be transferred instead to the plate. Thus, in may be preferable insome circumstances to have one or more fastener, more preferably two ormore fasteners remain substantially free to swivel or rotate to accountfor dimensional changes in the bone that may occur after insertion ofthe plate.

In some cases, however, it may be desirable to rigidly fix the angle ofthe fastener relative to the plate once it is deployed. While somedevices have been developed in the past to help resist or preventmicromovement of a plate relative to a fastener or to the bone that theplate contacts, past designs have either lacked the ability to beinserted at varying angles or have required complex designs oradditional components in order to achieve multi-angel variability.

One example is described in U.S. Pat. No. 4,484,570, which isincorporated herein in its entirety. In particular, this referencediscusses that reabsorption of the bone may take place at a portion ofthe contact surface between the bone and the plate. Over time, thisreabsorption can cause open gaps to be formed, which may eventuallybecome large enough that varying loads acting on the bone can causeundesirable micromovement between the plate, bone, and fasteners. Thisreference addresses this issue by describing a fastener having a headconfigured with a generally conical outer surface and having one or moreslots. The fastener head further has a clearance hole or receptacle inwhich an expanding set screw may be inserted to splay or direct portionsof the slotted head radially outwards. The interior surface of thefastener head is also generally conical and corresponds to a conicalouter surface of the set screw. Thus, as the set screw is driven furtherinto the clearance hole or receptacle, the interaction between the twoconical surfaces applies progressively greater amounts of locking force.As mentioned above, one disadvantage to the locking fastener systemdescribed in the '570 patent is that it is not capable of permittingadjustability of the fastener with respect to the plate.

Another example is found in U.S. Pat. No. 6,235,033, which also isincorporated herein in its entirety. In particular, the '033 patentpurports to achieve multi-angle variability of a plate design basedsubstantially upon the addition of a c-ring to the design described inthe '570 patent. In particular, the '033 patent likewise teaches to usea fastener having a slotted head with a generally conical outer surface.The conical surface of the fastener can be connected to a c-ring thatresides in the opening of the plate through which the fastener isinserted. The outer surface of the c-ring slidingly engages with thespherical surface of the hole in the plate to provide variation in theangle of the fastener. When desired, an expansion screw may be deployedin a receptacle formed in the fastener head so that the outer surface ofthe fastener head apply outward pressure against the c-ring. Eventually,the c-ring expands sufficiently to apply pressure against the opening inthe plate that locks the fastener relative to the plate. Onedisadvantage to this multi-angle locking system, however, is that itrequires that the plate be assembled with a c-ring in each opening orhole through which a fastener will be placed.

While any of the various methods and techniques described in thesereferences for having the fastener head capable of applying an outwardforce may be used, the present invention also relates to an improved wayto achieve multi-angle variability while preserving simplicity ofdesign. Rather than using a complex, multi-piece plate construction orsacrificing the ability of the fastener to have variable angles relativeto the plate, the present invention contemplates forming the outersurface of a slotted fastener head to have a curved or spherical shapecorresponding generally to the curvature of a portion of the plate holesin which the fastener will be placed. Thus, the fastener may be insertedinto a plate hole at a variety of angles and be selectively locked inposition without the use of a c-ring, bushing, or the like to aid inproviding multi-angle variability. Once the fastener is in its desiredposition, a set screw may be inserted into a receptacle in the fastenerhead to rigidly hold the fastener in a fixed position relative to theplate.

The outer surface of the curved fastener head may be textured to provideincreased locking forces. For example, a portion of the outer surface ofthe fastener head may be configured with circular grooves that help holdthe fastener in place as the slotted head is expanded outward againstthe inner surface of the plate hole. Likewise, the outer surface of thefastener may be roughened to provide increased resistance to slippagebetween the fastener head and the plate when in a rigid position.

FIGS. 15 A-C illustrate one example of a fastener of the presentinvention that is capable of selectively providing the ability to swivelor move and to hold a fixed position. In particular, the fastener headhas an outer surface that is generally spherical in shape, therebyallowing it to rotate or swivel once past the interference area. Thefastener head also has a plurality of slots or cuts in the head thatpermit the head to expand or compress. The fastener head also may havean interior space that is capable of securely receiving a secondfastener 1250, such as a set screw, a cam, or the like. As the secondfastener is inserted into this interior space, its causes the diameterof the first fastener head to increase and press against the inner wallof the bone screw opening in the plate, thereby locking it in place. Asstated above, the outer surface of the first fastener head may betextured to further increase the ability to rigidly hold the fastener inplace.

The inner space of the first fastener and outer shape of the secondfastener may have different configurations to create and apply a lockingforce. For example, in one embodiment the set screw, interior space, orboth may have a generally conical shape that progressively appliesgreater outward forces as the set screw or second fastener is inserted.Likewise, the interior space, second fastener head, or both may begenerally cylindrical with the diameter of the second fastener beinggreater than the inner diameter of the interior space.

Thus, in accordance with the present invention, the bone fixation platesand components described with reference to FIGS. 1-15 may be secured tovertebrae and other bony material in a manner that prevents the screwsfrom working loose when subject to vibration. Furthermore, theembodiments described above prevent the backing out of screws whileminimizing the profile of the bone fixation plate. Retaining features,provided near each opening through which a screw may pass, is moveablebetween relaxed and flexed positions. Another advantage of the presentinvention is that screws that fasten the plate to the bony tissue may beoriented at a variety of non-perpendicular angles with respect to theplate, which allows a relatively close spacing of fasteners without therisk of fasteners interfering with one another.

Although the present invention has been described with respect toseveral embodiments, it will be understood by those skilled in the artthat the present invention is capable of alternate embodiments withinthe spirit of the appended claims. For instance, while the embodimentsdescribed herein refer to a plate useful for the cervical region of thespine, skilled artisans would understand that the plate design describedherein may also be used in other regions of the spine or even forfixation of other bones in other parts of the body. Thus, the inventionis not limited only to treating the cervical spine.

What is claimed is:
 1. A fastener system for mounting an implant to bonecomprising: an elongate fastener having a longitudinal axis, a proximalend, and a distal end; a shaft portion including a threaded portiondisposed on the distal end of the fastener, wherein said threadedportion is capable of screwing into bone; a fastener head disposed onthe proximal end of the fastener, wherein the fastener head comprises: apartially spherical outer surface; an interior surface defining areceptacle for receiving a locking fastener, wherein the interiorsurface is non-threaded; and at least one slit extending between theouter surface and interior surface of the fastener head to permitoutward expansion of the fastener head, the at least one slit extendingexclusively along a length of the fastener head without entering theshaft portion; a locking fastener capable of being received in thereceptacle of the fastener head, wherein the locking fastener has adifferent configuration than the receptacle to create and apply alocking force; and a plate having at least one aperture including aninterference area integrally formed therein configured to prevent theelongate fastener from backing out of the interference area, wherein theat least one aperture has a spherical curvature extending at leastpartially through the thickness of the plate, wherein insertion of thefastener head interferes with the interference area, and wherein thefastener head is capable of engaging with and passing the interferencearea to communicate with the spherical curvature.
 2. The fastener systemof claim 1, wherein the fastener head comprises two or more slitsextending between the outer surface and interior surface of the fastenerhead to permit outward expansion of the fastener head.
 3. The fastenersystem of claim 1, wherein the fastener head comprises four or moreslits extending between the outer surface and interior surface of thefastener head to permit outward expansion of the fastener head.
 4. Thefastener system of claim 1, wherein the locking fastener is a cam. 5.The fastener system of claim 1, wherein the interior surface definingthe receptacle and an outer surface of the locking fastener are ofdifferent configurations to create a locking force.
 6. The fastenersystem of claim 1, wherein the interior surface defining the receptacleand an outer surface of the locking fastener are generally cylindricalwith the diameter of the locking fastener being greater than thediameter of the interior surface.
 7. The fastener system of claim 1,wherein the interference area includes alternating sections ofengagement and sections of relief.
 8. A fastener system for mounting animplant to bone comprising: an elongate fastener having a longitudinalaxis, a proximal end, and a distal end; a fastener head disposed on theproximal end of the fastener, wherein the fastener head comprises atleast one slit located on the fastener head to permit outward expansionof the fastener head, the at least one slit extending exclusively alonga length of the fastener head without entering a shaft portion, whereinthe fastener head further comprises an interior surface defining areceptacle for receiving a locking fastener, wherein the interiorsurface is non-threaded; a locking fastener capable of being received inthe receptacle formed in the fastener head, wherein the locking fastenerhas a different configuration than the receptacle to create and apply alocking force; and a plate having at least one aperture including aninterference area integrally formed therein configured to prevent theelongate fastener from backing out of the interference area, wherein theat least one aperture has a spherical curvature extending at leastpartially through the thickness of the plate, wherein insertion of thefastener head interferes with the plate at the interference area, andwherein the fastener head is capable of engaging with and passing theinterference area to communicate with the spherical curvature.
 9. Thefastener system of claim 8, wherein the fastener comprises a partiallyspherical outer surface.
 10. The fastener system of claim 8, wherein thelocking fastener comprises a cam.
 11. The fastener system of claim 8,wherein the fastener head comprises two or more slits extending betweenan outer surface and interior surface of the fastener head.
 12. Thefastener system of claim 8, wherein the fastener head comprises four ormore slits extending between an outer surface and interior surface ofthe fastener head.
 13. The fastener system of claim 8, wherein theinterior surface defining the receptacle in the fastener head and anouter surface of the locking fastener are of different configurations tocreate a locking force.
 14. A fastener system for mounting an implant tobone comprising: an elongate fastener having a longitudinal axis, aproximal end, and a distal end; a fastener head disposed on the proximalend of the fastener, wherein the fastener head comprises at least oneslit located on the fastener head to permit outward expansion of thefastener head, the at least one slit extending exclusively along alength of the fastener head without entering a shaft portion, whereinthe fastener head further comprises an interior surface defining areceptacle, wherein the interior surface is non-threaded; a lockingfastener capable of being received in the receptacle formed in thefastener head, wherein the locking fastener has a differentconfiguration than the receptacle to create and apply a locking force,and a plate having at least one aperture including an interference areaintegrally formed therein configured to prevent the elongate fastenerfrom backing out of the interference area, wherein the elongate fasteneris configured to interfere with the interference area, and wherein theinterference area includes a plurality of relief areas and a pluralityof engagement areas, wherein the interior surface defining thereceptacle in the fastener head and an outer surface of the lockingfastener are generally cylindrical with the diameter of the lockingfastener being greater than the diameter of the interior surface. 15.The fastener system of claim 14, wherein the locking fastener comprisesa cam.